Add GitHub actions workflow for updating draft

Add a workflow that will trigger on pushes to draft which:
* Builds the HTML version of the spec (using the Makefile) in a build
  subdirectory
* Moves the built HTML file as an index.html to a 'draft' subdirectory
* Switches to the gh-pages branch and moves the contents of the build
  subdirectory into the repo root
* Commits the generated HTML files and pushes them to the gh-pages branch
  on GitHub

Signed-off-by: Joshua Lock <[email protected]>

Author: joshuagl

.github/workflows/draft.yml

@@ -0,0 +1,53 @@
+name: Update draft specification
+on:
+  workflow_dispatch:
+  push:
+    branches: draft
+
+jobs:
+  build-draft:
+    name: Make draft spec release
+    runs-on: ubuntu-latest
+    steps:
+    - name: Set up Python
+      uses: actions/setup-python@3105fb18c05ddd93efea5f9e0bef7a03a6e9e7df
+      with:
+        python-version: 3.x
+
+    - name: Find pip cache dir
+      id: pip-cache
+      run: echo "::set-output name=dir::$(pip cache dir)"
+
+    - name: pip cache
+      uses: actions/cache@26968a09c0ea4f3e233fdddbafd1166051a095f6
+      with:
+        # Use the os dependent pip cache directory found above
+        path: ${{ steps.pip-cache.outputs.dir }}
+        # A match with 'restore-keys' is used as fallback
+        key: ${{ runner.os }}-pip-
+
+    - name: Clone
+      uses: actions/checkout@5a4ac9002d0be2fb38bd78e4b4dbde5606d7042f
+
+    - name: Build specification
+      run: |
+        python -m pip install bikeshed
+        mkdir build && cd build
+        make -f ../Makefile draft
+
+    - name: Switch branch
+      uses: actions/checkout@5a4ac9002d0be2fb38bd78e4b4dbde5606d7042f
+      with:
+        ref: gh-pages
+        clean: false
+
+    - name: Push generated specification
+      run: |
+        git config user.name "TUF Specification Automation"
+        git config user.email [email protected]
+        rm -fr draft
+        mv build/* .
+        rmdir build
+        git add .
+        git commit -m "Publish latest specification"
+        git push